Small engine repair - Snowmobile?

[Kenneth M]

I recently did a top half motor rebuild on a 3 cylinder liquid cooled snowmobile (1996 polaris XCR 600 SP). I got the machine running and it seemed to run fine when i did a short test drive on 2 occassions. on the third attempt, i added a little more anti-freeze to the resevoir as it was a liitle low. when I pulled on the starter cord it was a little hard to pull but eventually it started. It seemed a little boggy but i didn't run it long when i noticed that the anti freeze was pouring out of the overflow tube. i shut it off and a few minutes later opened the resevoir cap, knowing it was still cool, and anti freeze came blowing out of there. anybody have any idea why there was so much pressure and why the engine was hard to pull over? is the anti freeze going somewhere in the engine block where it is not suppose to. what is my next step to check? thank you.